Table 1. Land cover class descriptions for the Upper San Pedro Watershed

Forest

Vegetative communities comprised principally of
trees potentially over 10m in height and typically characterized
by closed or multi-layered canopies. Species in this category are
evergreen (with the exception of aspen), largely coniferous (e.g.
ponderosa pine), and restricted to the upper elevations of
mountains that arise off the desert floor.

Oak Woodland

Vegetative communities dominated (>30% total
cover) by evergreen trees (Quercus spp.) with a mean
height usually between 6 and 15m. Tree canopy is usually open or
interrupted and singularly layered. This cover type often grades
into forests at its upper boundary and into semi-arid grassland
below.

Mesquite Woodland

Vegetative communities dominated by leguminous
trees (Prosopis spp.) whose crowns cover 15% or more of
the ground often resulting in dense thickets (30-75% total cover).
Historically maintained maximum development on alluvium of old
dissected flood plains; now present without proximity to major
watercourses. Winter deciduous and generally found at elevations
below 1,200m.

Grassland

Vegetative communities dominated by perennial
and annual grasses (>35% total cover) with occasional
herbaceous species present. Trees and shrubs do not exceed 20% of
the total cover. Generally grass height is under 1m and they occur
at elevations between 1,100 and 1,700m; sometimes as high as
1,900m. This is a landscape largely dominated by perennial bunch
grasses separated by intervening bare ground (45-50% total cover)
or low-growing sod grasses and annual grasses with a
less-interrupted canopy. Semi-arid grasslands are generally
positioned in elevation between evergreen woodland above and
desertscrub below.

Desertscrub

Vegetative communities comprised of short shrubs
with sparse foliage (>35% total cover) and small cacti that
occur between 700 and 1,500m in elevation. Within the San Pedro
river basin this community is often dominated by one of at least
three species, i.e. creosotebush, tarbush, and whitethorn acacia.
Individual plants are often separated by significant areas of
barren ground (40-45% total cover) devoid of perennial vegetation.
Many desertscrub species are drought-deciduous.

Riparian

Vegetative communities adjacent to perennial and
intermittent stream reaches. Trees can potentially exceed an
overstory height of 10m and are frequently characterized by closed
or multi-layered canopies depending on regeneration. Species
within the San Pedro basin are largely dominated by two species,
i.e. cottonwood and Goodding willow. Riparian species are largely
winter deciduous.

Agriculture

Crops actively cultivated (and irrigated). In
the San Pedro River basin these are primarily found along the
upper terraces of the riparian corridor and are dominated by hay
and alfalfa. They are minimally represented in overall extent
(less than 3% total cover) within the basin and are irrigated by
ground and pivot-sprinkler systems.

UrbanLow Density
High Density

This is a land-use dominated by
small ejidos
(farming villages or communes), retirement homes, or
residential
neighborhoods (Sierra Vista). Heavy industry is represented by a
single open-pit copper mining district near the headwaters of the
San Pedro River at Cananea, Sonora (Mexico).

Water

Sparse free-standing water is available in the
watershed. This category would be mostly represented by perennial
reaches of the San Pedro and Babocomari rivers with some attached
pools or repressos (earthen reservoirs), tailings ponds near
Cananea, ponds near recreational sites such as parks and golf
courses, and sewage treatment ponds east of the city of Sierra
Vista, Arizona.

Barren

A cover class represented by large rock
outcropping or active and abandoned mines (including tailings)
that are largely absent of above-ground vegetation.